Method for the depolymerization of polytetrafluoroethylene

ABSTRACT

High yield monomeric tetrafluoroethylene may be recovered from polytetrafluoroethylene and polytetrafluoroethylene composites by pyrolyzing the polymer in the presence of high temperature steam. The steam acts as a condensable carrier gas for the low molecular weight pyrolysis products and should be present in an amount such that the mole ratio of steam to pyrolysis products is at least one to one. The steam also acts as a means for heating the polymer to the necessary decomposition temperatures.

United States Patent [191 Arkles et al.

m1 3,832,411 1451 Aug. 27, 1974 METHOD FOR THE DEPOLYMERIZATION OFPOLY'IETRAFLUOROETHYLENE [75] Inventors: Barry C. Arkles, Malvem; RobertN.

Bonnett, West Chester, both of Pa.

[73] Assignee: Liquid Nitrogen Processing Corporation, Malvem, Pa.

[22] Filed: Feb. 2, 1971 [2]] Appl. N0.: 111,963

ZONE

A PYROLYSIS PRODUCTS 960,309 6/1964 Great Britain 260/6533 PrimaryExaminerLeon Zitver Assistant ExaminerJoseph A. Boska Attorney, Agent,or Firm-Seidel, Gonda & Goldhammer [57] ABSTRACT High yield monomerictetrafluoroethylene may be recovered from polytetrafluoroethylene andpolytetrafluoroethylene composites by pyrolyzing the polymer in thepresence of high temperature steam. The steam acts as a condensablecarrier gas for the low molecular weight pyrolysis products and shouldbe present in an amount such that the mole ratio of steam to pyrolysisproducts is at least one to one. The steam also acts as a means forheating the polymer to the necessary decomposition temperatures.

8 Claims, 2 Drawing Figures GASEOUS 54 PYROLYSIS 60 CONDENSED STEAM 63CONDENSED PYROLYSIS PRODUCTS METHOD FOR THE DEPOLYMERIZATION OFPOLYTETRAFLUOROETHYLENE The present invention relates to the productionof low molecular weight fluorine-containing compounds by depolymerizingpolymeric tetrafluoroethylene. More particularly, the invention isdirected to a method of recovering monomeric tetrafluoroethylene frompolymeric tetrafluoroethylene by pyrolysis using the controlled presenceof high temperature steam.

The pyrolysis of polymeric tetrafluoroethylene (generally referred to aspolytetrafluoroethylene, TFE fluorocarbon polymer or PTFE) under varyingconditions, including in air and under vacuum, is well known in the art.See for example S. L. Madorsky, Thermal Degradation of Organic Polymers,pp. 130 et seq., Wiley (1964). The pyrolysis products are a mixture ofmany compounds including -TFE monomer, hexafluoropropene (C Foctafluorocyclobutane (C F and other gaseous and liquid perfluorinatedproducts having relatively low boiling points. When pyrolysis isconducted at near atmospheric pressures these products usually containconsiderably less than 50 percent of the monomer, and consequently themixture is of little commercial value.

Other attempts have been made to enhance the yield of monomertetrafluoroethylene by decreasing residence time at pyrolytictemperatures by means of evacuating the pyrolysis zone to subatmosphericpressure. See for example US. Pat. No. 2,406,153 to E. E. Lewis. Thelatter patent discloses yields of monomeric tetrafluoroethylene as highas 85 percent based on the gaseous products alone when pressures werereduced below 150 millimeters of mercury. Although this represented avast improvement in yield, the process cannot be satisfactorily employedfor several reasons. Although it is not difficult to produce a vacuum ina vessel containing polytetrafluoroethylene, it is quite difficult topyrolyze large quantities since the vessel must ordinarily be heatedexternally. The difficulty arises because: (1) materials capable ofwithstanding the high temperature and corrosive pyrolysis conditionsgenerally have poor strength and rigidity at the high temperaturesinvolved; (2) the use of vacuum precludes convective heat supply to thepolymer and thus restricts the supply of en'- ergy necessary forpyrolysis to conduction and radiation from the surface of the vessel;and (3) polytetrafluoroethylene itself has very poor heat-transferproperties. The net result is that the maximum diameter of avacuum-pyrolysis vessel is generally limited to about 10 inches, and thebatch of polymer to be pyrolyzed is generally restricted to a relativelysmall weight. Furthermore, vacuum pyrolysis offers the probability ofhaz ardous introduction of air into the pyrolyzate stream. As a result,to the best of our knowledge no important commercial depolymerization ofpolytetrafluoroethylene in vacuum has been accomplished.

Scrap or waste polytetrafluoroethylene is an attractive source ofmonomer tetrafluoroethylene from several standpoints. In economic terms,the production of monomer tetrafluoroethylene by depolymerization of thepolymer represents a substantial cost saving compared to the synthesisof the monomer from usual refrigerant gas sources. In process terms, asubstantial difficulty encountered in producing polytetrafluoroethyleneis the presence of by-product difluoroethylene and trifluoroethylenewith the crude monomer tetrafluoroethylene made from CHCIF or CHF Theseimpurities are generally removed by slow and costly methods. On theother hand, it is believed that monomer from the depolymerization ofpolytetrafluoroethylene has substantially reduced portions of theseproducts.

Additionally, the depolymerization process is attractive from anenvironmental standpoint. Mechanical reprocessing ofpolytetrafluoroethylene is practiced only to a small extent due to theloss in mechanical properties and processability of the reprocessedpolymer. Therefore, disposal of waste polymer is resulting increasinglyin an accumulation of non-biodegradeable polytetrafluoroethylene. Thus,production of monomer tetrafluoroethylene from scrap polymer results ina beneficial recycling of material the accumulation of which ispotentially disadvantageous to the environment.

Accordingly, it is an object of the present invention to develop acommercially feasible process for the production of tetrafluoroethylenemonomer from polytetrafluoroethylene.

It is a further object of the present invention to produce high yieldsof tetrafluoroethylene monomer by the depolymerization ofpolytetrafluoroethylene.

It is a still further object of the present invention to produce goodyields of other low molecular weight fluorine-containing compounds, suchas commercially valuable hexafluoropropylene.

It is another object of the present invention to provide a method ofdepolymerizing polytetrafluoroethylene in any form without the necessityof using subatmospheric pressures.

It is still another object of the present invention to provide a methodof recycling non-biodegradeable polytetrafluoroethylene whether it be inclean, contaminated or composite form.

Other objects will appear hereinafter.

The objects of this invention are achieved by heating polymerictetrafluoroethylene to temperatures above the decomposition temperatureof the polymer in the presence of high temperature steam under suchconditions that the mole ratio of steam to off-gas decompositionproducts is at least one to one. The off-gas products are collectedafter condensing the steam.

For the purpose of illustrating the invention, there are shown in thedrawings forms which are presently preferred; it being understood,however, that this invention is not limited to the precise arrangementsand instrumentalities shown.

FIG. 1 is a simplified representation of an apparatus which may be usedin the present invention according to Example 1.

FIG. 2 is a simplified representation of an apparatus which may be usedfor processing larger quantities according to the method of the presentinvention.

The process of the present invention depends in large part upon the roleof steam as a diluent and carrier gas for the tetrafluoroethylenemonomer, which is the predominant product of the decomposition of thepolymer. In addition, the commercial feasibility of the presentinvention depends upon the fact that high temperature steam mayconveniently be used to conduct sufficient heat to thepolytetrafluoroethylene to decompose the material.

While thermal decomposition of polytetrafluoroethylene is observed aslow as 450 F, the decomposition at this temperature'only proceeds at arate on the order of percent per hour. It is not until considerablyabove the polymer first-order transition temperature (approximately 625F) that substantial decomposition is observed. That is, thepolytetrafluoroethylene should be heated to a temperature of betweenabout 780 F and l,400 F, and preferably between about 800 F and I,IOO F.Good results may be achieved when the temperature of the steam impingingupon the polymer melt is between 950 F and l,800 F.

Although the specific temperature of the steam supplied to the polymertetrafluoroethylene mass may be varied over a large range, the flow rateof steam required will be largely dependent on the temperature of thesteam. This is due to the fact that the depolymerization rate isincreased by increased interface temperature of the polymer underpyrolysis. Therefore, as the temperature is raised, the mass-rate ofsteam must be increased in order to adequately dilute thetetrafluoroethylene monomer which is forming at a more rapid rate, andthereby minimize the tendency of free-radical difluoromethylene groupsto trimerize or polymerize. In other words, to maintain a high degree ofTFE monomer yield, it is prudent to dilute free-radical interaction bymaintaining an adequate steam flow rate. In general, the higher thetemperature, the more steam required to maintain uniform yield.

An easy way to accomplish uniform yield is to maintain a constant moleratio of steam to off-gas decomposition products. It has been found thata minimum mole ratio of steam to decomposition products of about four toone is required to maintain a high level of monomer yield. Mole ratiosof as low as about one to one still produce good levels of monomer'yieldas well as increased yields of other low molecular weightfluorinecontaining compounds. At lower mole ratios, monomertetrafluoroethylene concentration is low, and the amount of solidsublimate as well as other decomposition by-products is increased. Onthe other hand, at extremely high mole ratios, it is possible to producealmost pure monomer tetrafluoroethylene. A mole ratio of steam todecomposition products of between about ten to one and forty to one ispreferable. There is apparently no upper limit on the amount of steamwhich may be used or the maximum temperature of the steam. However,economic considerations render higher temperatures and amountsimpractical.

The commercial depolymerization of polytetrafluoroethylene may becarried out in vessels of various configurations. A simplifiedrepresentation of one such vessel is illustrated in FIG. 2 of theaccompanying drawings.

The reaction vessel 10 should be made of a material which is able towithstand the high temperatures and corrosive byproducts of thepyrolysis reaction. An optimum material is platinum lined hightemperature stainless steel. However, lnconel (a trademark for alloys ofnickel and chromium) or Monel metal (a trademark for alloys ofpredominantly nickel and copper) may also be employed. It is possible touse stainless steel or iron, but these material are less desirable.

The high temperature steam used in the process of the present inventionmay be produced either outside or within the reaction vessel.Super-heated steam, steam or water enters the vessel 10 through inlettube 12 at the side of the vessel near the bottom. Inlet tube 12 extendsapproximately to the center of the vessel.

The tube. may be made of stainless steel, for example.

In order to produce the steam or maintain or raise the temperature ofthe steam, the vessel 10 is provided with gas burners 14. In addition,the sides and top of the vessel are provided with suitable insulation16.

The batch 18 of material. to be pyrolyzed is supported by a poroussurface 20 which may be made of a heavy gaugescreen or a perforatedmetal plate. The porous surface 20 is in turn supported by a replaceabletripod 22, which is positioned over the steam inlet tube 12.

The screen or perforated metal plate which forms the porous surface 20not only serves as a support for batch 18, but also conducts heat to thebatch to further the decomposition. Since typically encountered PTFEmelt does not usually flow until it reaches a temperature of about 850 For more, flowing of the melt through the holes of the screen orperforated plate is not a major problem. In fact, the melt produces astalactite effect, and the hotter steam'beneath the screen or platedecomposes the polymer before the drips of 'melt can hit the bottom ofthe vessel. Nevertheless, the holes in the screen or plate should besufficiently small to prevent particles of the solid polymer fromfalling through. For example, two overlapped layers of 14 mesh screenhave been found to be quite suitable, whereas 8 mesh screen or inchdrilled holes in a steel plate were usually'too large.

It has also been found-preferable to cover or wrap the batch 18 withscreen 24, which may suitably bemade of 14 mesh stainless steel. Thescreen 24 inhibits the blowing off of any solid raw-material with thesteam and decomposition gases, as well as reducing sublimate formation.The reduction of sublimation is important since the condensation of thesublimate tends to block the exit gas lines. As already mentioned,sublimate formation may also be reduced or substantially eliminated byusing higher mole ratios of steam.

After passing through .and around the porous supporting surface 20 andthe batch 18, the steam carries the gaseous pyrolysis products away fromthe polymer mass and out of the vessel 10 through effluent gas outlet26, which may suitably be stainless steel tubing. It is good practice toposition the effluent outlet 26 below the vessel opening 28, sincegasketing capable of withstanding the depolymerization temperatures andcorrosive conditions (i.e., hydrofluoric acid) is generally notavailable. For example, in a vessel having a volume of 8 liters theefiluent gas outlet 26 may suitably be positioned about 2 inches abovethe batch 18 and 6 inches below the vessel opening 28. Also, the lid 30of the vessel may be gasketed with polytetrafluoroethylene, providedenough of the heat supplied to the bottom of the vessel is removedthrough effluent outlet 26 to keep the gasket temperature below about500 F.

In order to aid the control of the steam temperature, the vessel may besuitably provided with thermocouples 32, in wells 33 positioned in thelid 30 and the bottom 34 of vessel 10, for example, to determine therespective temperatures of the batch l8 and the steam below the batch.The vessel may also be provided with appropriate flowmeters (not shown)at the inlet tube 12 and/or outlet 26 to read the amounts of steam andpyrolyzate passing through the system.

Following the effluent gas outlet 26 is a heat exchanger and waterdrop-out container (not shown) for condensing the steam. This apparatusmay be of any EXAMPLE I This example was performed in the apparatuswhich is illustrated in the simplified representation of FIG. 1 of theaccompanying drawings. I

A 100 gram sample 40 of unfilled polytetrafluoroethylene resin waswrapped in stainless steel screening 42 and placed one foot from the endof a 3% foot long stainless steel tube 44 having a diameter of one inch.The 2 foot length of tubing 44 prior to the polytetrafluoroethylenesample 40 was wrapped with heating bands 46. The entire length of tube44, except for the last 6 inches on the end 48 closest to the sample 40,was provided with insulation 50. The end 48 was reduced to A inchdiameter stainless steel tubing 52 which ran for 3 feet, acting as anair condenser, which in turn was connected to a rubber tube 54 which ledto a small filter flask 56 for dropping out the water 58 condensed fromthe steam. Filter flask 56 was provided with a product gas outlet 60which led to a collection flask 62. Collection flask 62 was immersed ina dry ice-acetone bath 63 for condensation of the decomposition gases.

Steam entered end 64 of the reaction tube 44 after being boiled from a2,000 milliliter flash (not shown). Over a one half hour period, 250grams of steam were produced. A thermocouple (not shown) between theinsulation and the outside of the tube indicated a temperature of 850 Fat the site of the polymer. The mole ratio of steam to effluent gasproducts was approximately 60 to l, and the amount of material pyrolyzedwas approximately 21 grams. Gas samples were taken from the gas outlet60 between filter flask 56 and collection flask 62 and analyzed.

The analysis showed the effluent gas to contain 98 percenttetrafluoroethylene monomer (C F 1.25% C F and 0.75% C F,,. No Sublimatewas produced.

EXAMPLE ll Steam at a temperature of 500 F was introduced at a rate of7-l0 grams per minute to the bottom of an 8 liter stainless steel vesselsimilar to that shown in FIG. 2 of the drawings. The base of the vesselwas heated by the gas burners to raise the steam temperature to aboutl,l00 F. The temperature at the bottom of the polymer was 950 F. Priorto the introduction of the steam, a 1 pound sample ofpolytetrafluoroethylene was loaded into the vessel. Under the action ofthe steam the resin was depolymerized over a one hour period and yieldedan off gas concentration of 70 percent tetrafluoroethylene monomer.

EXAMPLE 111 One kilogram of unsintered scrap PT FE containing 25 percentfiber glass and pigmentation was pyrolyzed in the same vessel as Examplell with l,l00 F steam flowing at a rate of 12-15 grams per minute. Overa one half hour period, 250 grams of the material was pyrolyzed. Themole ratio of steam to off gas products was approximately 10 to l. Thecomposition of the product gases was QB, 10% C F and 4% C F EXAMPLE IV Apyrolysis run was made in the same vessel as Example I! using sintered,unfilled scrap PT FE. Using l,l00 F steam at a rate of 15-20 grams perminute, a 500 gram sample of the scrap PTFE was completely pyrolyzedover a one hour period. The unrefined off gases were composed of percenttetrafluoroethylene monomer.

EXAMPLE V A l kilogram sample of PT F E material was pyrolyzed in theapparatus of Example II with l,l00 F steam flowing at 20-25 grams perminute. The PTF E material employed consisted of whole and brokensintered billets filled with bronze, carbon, molybdenum disulfide andsome pigmentation. Over a minute period the sample was pyrolyzedcompletely and produced a monomer tetrafluoroethylene yield (based ongases) of 70-85 percent. Sublimate formation was reduced with thishighsteam flow to about 5 percent of the total solids.

EXAMPLE VI The pyrolysis vessel of Example II was charged with 7 poundsof flake reprocess grade (scrap and turnings of PTFE put through agrinder to make uniform size sintered particles)polytetrafiuoroethylene. Steam having a temperature ranging between1,100 F and l,200 F was introduced at a rate of 35-40 grams per minute.The pressure measured at the bottom of the polymer melt was 5-8 psiduring equilibrium pyrolysis conditions. The flow of off gases wasmeasured after condensation of the steam, drying of the products andcondensation of products heavier than TF E. The TF E gas was produced atabout 2,200 cc per minute (approximately 10 grams per minute). Thisrepresented a mole ratio of steam to off gas products of approximately20 to l. The analysis of the effluent gases indicated 90-95 percentmonomer tetrafluoroethylene.

It will be readily understood by one of ordinary skill in the art thatthe above examples are only illustrative, and the exact processparameters can be varied widely. For example, the form of the apparatusmay be varied in order to operate the process of the present inventionin batches, semi-continuously, or continuously. Also, processes for theautomatic feed of polytetrafluoroethylene and PTFE composites into thepyrolysis vessel are conceivable, as are processes for withdrawal of thenon-decomposed inorganic fillers from the pyrolyzed composites.

The utility of the present invention is apparent when compared toprevious processes which either yield extremely low concentrations ofdesirable pyrolysis products or have limited commercial success becauseof restricted capacity. The process of this invention offers a highlyavailable feed stock for the production of tetrafluoroethylene polymerand fluorine containing compounds of various molecular weights.Furthermore, the invention can recycle scrap polytetrafluoroethylene invirtually any form, including scrap lathe-turnings and other machinescrap, unsintered or sintered tape and sheet trimmings, discardedfinished parts, and other unfilled or composite forms.Polytetrafluoroethylene composites may include, for example, mixtures ofPTFE with fiber glass, bronze, carbon and other inert materials.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification as indicating the scope of theinvention.

We claim:

1. A method of producing tetrafluoroethylene monomer by pyrolysis ofpolymeric tetrafluoroethylene, comprising the steps of heating polymerictetrafluoroethylene to a temperature of at least about 780 F. at apressure of at least atmospheric pressure and in the presence of hightemperature steam, said steam being present in an amount such that themole ratio of steam to pyrolysis products is at least about 4 to l,condensing the steam from the mixture of pyrolysis products of thepolymeric tetrafluoroethylene and collecting the resulting pyrolysisproducts.

2. A method according to claim 1 wherein the polymerictetrafluoroethylene is heated to a temperature of between about 780F.and 1,400F.

3. A method according to claim 1 wherein the polymerictetrafluoroethylene is heated to a temperature of between about 800F.and 1,100F., and the mole ratio of steam to said pyrolysis products isbetween about 10 to l and 40 to l.

4. A method according to claim 1 wherein the polymerictetrafluoroethylene is heated by steam having a temperature of at least950F.

5. In a method for depolymerizing polymeric tetrafluoroethylene bypyrolysis, the improvement comprising heating polymerictetrafluoroethylene at a pressure of at least atmospheric pressure andin the presence of high temperature steam sufficient to heat saidpolymeric tetrafluoroethylene to a temperature of at least about 780 F.,said steam being present in an amount such that the mole ratio of steamto pyrolysis products is at least about 4 to l, condensing the steamfrom the mixture of pyrolysis products, and collecting the resultingpyrolysis products.

6. A method according to claim 5 wherein said steam has a temperature ofbetween about 950F. and 1,800F.

7. A method according to claim 5 wherein the mole ratio of steam to saidpyrolysis products is between about 10 to 1 and 40 to l.

8. A method according to claim 5 wherein said polymerictetrafluoroethylene is recycled scrap containing fillers.

2. A method according to claim 1 wherein the polymerictetrafluoroethylene is heated to a temperature of between about 780*F.and 1,400*F.
 3. A method according to claim 1 wherein the polymerictetrafluoroethylene is heated to a temperature of between about 800*F.and 1,100*F., and the mole ratio of steam to said pyrolysis products isbetween about 10 to 1 and 40 to
 1. 4. A method according to claim 1wherein the polymeric tetrafluoroethylene is heated by steam having atemperature of at least 950*F.
 5. In a method for depolymerizingpolymeric tetrafluoroethylene by pyrolysis, the improvement comprisingheating polymeric tetrafluoroethylene at a pressure of at leastatmospheric pressure and in the presence of high temperature steamsufficient to heat said polymeric tetrafluoroethylene to a temperatureof at least about 780* F., said steam being present in an amount suchthat the mole ratio of steam to pyrolysis products is at least about 4to 1, condensing the steam froM the mixture of pyrolysis products, andcollecting the resulting pyrolysis products.
 6. A method according toclaim 5 wherein said steam has a temperature of between about 950*F. and1,800*F.
 7. A method according to claim 5 wherein the mole ratio ofsteam to said pyrolysis products is between about 10 to 1 and 40 to 1.8. A method according to claim 5 wherein said polymerictetrafluoroethylene is recycled scrap containing fillers.